From
Tuning Tips using the TMO Datalogger on VFAQ.com...
quote:
Duty cycle is roughly defined as the ratio of the amount of time a signal is "active" to the amount of time available for the signal to be active. A couple examples. In the examples that follow, the amount of time available for the signal is 20ms (milliseconds).
Signal is never active = 0ms/20ms= 0% duty cycle
Signal is always active = 20ms/20ms = 100% duty cycle
Signal is active for 8ms = 8ms/20ms = 40% duty cycle
Now, for the question of injector duty cycle. Well, the logger spits out how long the injectors are being opened in ms. What we need is the time available to the injector.
One might think the max amount of fuel an injector can squirt into the cylinder would be limited to the time that the intake valve is open. But that is a very short time relative to the entire four-stroke time that is really available. The injector can squirt fuel onto the back of the injector while it is closed and let it pile up there. This is actually done almost all the time at RPMs over 5000 (maybe lower, too) and is the reason why the intake valves get all crudded up over time with an accumulation of carbon. This is also why you should check and clean your intake valves occasionally.
Anyway, if we decide to use all four cycles of the engine and call that the max time period, we need to find the period of four cycles. That is simply equal to 1/RPMs times two, because it takes two rotations of the crankshaft to complete four cycles of the engine (1/RPMs because period is the inverse of frequency).
An example - what is the time available for the injector at 6000 RPM? Now, charting the time in terms of minutes just isn't convenient, so we first convert RPMs to rotations per second.
6000 RPM = 100 rot/sec
Inverting and multiplying by two, we get
0.010 seconds * 2 = 20ms
So, at 6000 RPMs, the time available to your injectors is only 20ms. But that isn't the whole story - it gets worse. There is a thing called injector dead time. This is the time it takes for your injector to respond to an "open-sesame" command from the ECU. It gets complicated because this time varies with the voltage at the battery. The ECU has a lookup table of dead-time vs. battery voltage and really is the main reason the ECU looks at battery voltage in the first place (there are other less important reasons). This time can range from 0.65ms to 2.1ms. Anyway, you have to subtract this from the time available to get the true time available to the injector.
Note: subtracted from this dead time is the time it takes to turn off the injector. But, when people talk of injector duty cycle, they never include this time. Well, they do, but they say things like, "Never run an injector over 80% (or 90%)." But such a thing is really kind of silly to say, as duty cycle is a ratio that doesn't take into account the times involved.
For example, would you want to run at 90% duty cycle at 2000 RPM?
1(2000/60) * 2 = 60ms
90% of 60ms = 54ms
Now, 54ms of fuel would probably be ridiculous for most applications. But I'm just using it as an extreme example. 54ms would probably also burn out the injector - I do not think you are supposed to keep them on that long. They do have a limit. You need to watch for that, too.
It just so happens that at 6000 RPM, 90% gives you 18ms, which leaves you 2ms of available "dead time", which is probably a good thing. At 8000, you only 15ms available, so 90% gives you a paltry 13.5ms (dead time of 1.5ms still probably okay on a good battery).
Of course, what is missing from this is what does 13.5ms mean, in terms of fuel? Not easy on the first generation, because 450cc/min injectors are further derated by the 1G low fuel pressure. That 450 rating is at 42.7 psi, not the 36.3 psi that the 1G fuel regulator uses. 450cc/min * sqrt(36.3/42.7) = 415cc/min injectors! Multiply this by 13.5ms = 0.093375cc of fuel. You need to figure out if this is enough for the air you are pushing through the engine.
Multiply this number by, say 11:1 A/F mass ratio and the mass of 1cc of fuel, and you get the mass of the air this amount of fuel will support. Now, if I had a way to flow-bench the MAF, I could figure out what the actual air-mass/sec rate is, and I could log it. And you could figure out if you are getting enough fuel into your engine to match the air.
Either that, or you could look at your O2 sensor readings!
BTW, by using the injector pulse width number along with the RPM value, I should be able to produce a nice duty cycle plot. I will add this data type in the future. But keep in mind what it really means. Personally, I find pulse width a hell of a lot more useful because it directly correlates to the amount of fuel entering the engine - duty cycle does not. -todd-
I have personally run my 450s on my 1g up to 27+ms and have a calculated duty cycle of 130% from 4K to 7500rpms. This seems confusing but it works. If you look at all the available charts and formulas we as DSMers may be running our injectors way too hard on PUMP gas.
450cc's @ 80% = 275hp, @ 100% = 342hp
550cc's @ 80% = 335hp, @ 100% = 419hp
650cc's @ 80% = 396hp, @ 100% = 495hp
720cc's @ 80% = 439hp, @ 100% = 548hp
This is using a B.S.F.C (Brake Specific Fuel Consumption) of .50lbs/hr/hp (that is .50lbs of fuel per hour for every horsepower it produces). It is recommended turbocharged engines run a B.S.F.C. of .60 or more which means even lower numbers. This is with PUMP gas. If you run race gas you can go with a lower B.S.F.C. but who knows how low, maybe .40 or .45. As you can see, if you are trying to run fast times off of 550's and PUMP gas you are probably running them too hard. Most people are probably running their 450's and 550s too hard on the street and are running them past 80% duty cycle. Is this a bad thing? Maybe, but you can't argue with success.
The one thing I will add is at WOT you want a value to be a smooth, straight curve. If the value is jumpy at all your injectors are not squirting a smooth pattern and you will get detonation from this. If you add fuel through your VPC, AFC etc. and you notice the injector open time did not increase at high rpms, you might be in need of bigger injectors as you're probably pushing the ones you have to the max.
